Java Null behaviour

Question

I am trying to understand how null works in Java.

If we assign null to any object, what happens actually behind the scene? Does it assign a memory location address pointing to a null "object" or something else?

I've tried the following program and I've come to understand that all nulls point to same location.

But can anybody tell me how Java throws NullPointerException and how null works in Java?

class Animal{
}

class Dog{
}


public class testItClass {

    public static void main(String args[]){
        Animal animal=null;
        Dog dog=null;

        if(((Object)dog) == ((Object)animal))
            System.out.println("Equal");
    }

}

Output

Equal.

Answer 1

if we assign null to any object what it actually is it some memory location in heap OR anything else.

One should distinguish reference and object. You can assign null to a reference. Objects are normally created in heap using new operator. It returns you a reference to an object.

A a = new A();

object with type A is created in heap. You are given back reference a. If now you assign

a = null;

the object itself still reside in heap, but you would not be able to access it using reference a.

Note that object might be garbage collected later.

UPD:

I have created this class to see byte code of it (first time to me):

public class NullTest {
    public static void main (String[] args) {
        Object o = new Object();
        o = null;
        o.notifyAll();
    }
}

And it produces:

C:\Users\Nikolay\workspace\TestNull\bin>javap -c NullTest.class
Compiled from "NullTest.java"
public class NullTest {
  public NullTest();
    Code:
       0: aload_0
       1: invokespecial #8                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #3                  // class java/lang/Object
       3: dup
       4: invokespecial #8                  // Method java/lang/Object."<init>":()V
       7: astore_1
       8: aconst_null
       9: astore_1
      10: aload_1
      11: invokevirtual #16                 // Method java/lang/Object.notifyAll:()V
      14: return
}

You can see that set null to a reference results:

8: aconst_null
9: astore_1

List of byte code instructions

Basically it puts value of null to the top of stack and then saves to the reference. But this mechanism and reference implementation is internal to JVM.

How is reference to java object is implemented?